Switching power supply

1. Field of the Invention

The present invention relates to a switching power supply, and more particularly, the present invention relates to a switching power supply that detects voltage for protecting individual functional circuits from experiencing an overcurrent.

2. Description of the Related Art

In a series-connected voltage stabilizer, when the load of a constant-voltage power supply is short-circuited or overloaded, a switching transistor for control or other function can be damaged. Thus, an overcurrent protection function is provided in the switching power supply to supply the power to individual functional circuits.

FIG. 2 is a circuit diagram showing a conventional switching power supply. A switching power supply 50 has been disclosed in Japanese Unexamined Patent Application Publication No. 6-269159, and includes a switching element 51 , a diode D 51 , choke coils L 51 and L 52 , a comparator-detector 52 , and a drive circuit 53 .

The switching element 51 is switched on or switched off according to the differential voltage between the input voltage Vi and the output voltage Vo. The diode D 51 is a flywheel diode constructed to dissipate the energy (the power) accumulated in the choke coil L 51 when the switching element 51 is switched on if the switching element 51 is switched off. The choke coil L 51 accumulates the pulse voltage applied when the switching element 51 is switched on as energy. The choke coil L 52 reduces ripple noise and spike noise. The comparator-detector 52 is inserted between both ends of the choke coil L 52 , detects the current flowing in the choke coil L 52 , and controls the drive circuit 53 which sets the ON-OFF ratio of the switching element 51 . The drive circuit 53 outputs the control signal at the pulse so that the output voltage applied to the load (not shown in the figure) is consistent with the input voltage, implements the ON-OFF control of the switching element 51 , and detects overcurrent.

As described above, the current is detected by making use of a resistance component of the choke coil L 52 , and overcurrent protection is realized in a configuration in which the comparator-detector 52 is inserted between both ends of the choke coil L 52 .

However, since the conventional switching power-supply detects the current by making use of the resistance component of the choke coil in order to reduce ripple noise and spike noise and realizes overcurrent protection, problems occur in that two choke coils, i.e., one choke coil provided for accumulating the pulse voltage applied when the switching element is switched on as energy and another choke coil provided for detecting the current are necessary, the manufacturing cost is increased, and the size of the switching power supply is increased.

In order to overcome the problems described above, preferred embodiments of the present invention provide a switching power supply in which the manufacturing cost and the size thereof are greatly reduced while solving the problems described above.

According to a preferred embodiment of the present invention, a switching power supply includes a switching element, a choke coil and a comparator-detector, wherein first and second voltages are detected from both ends of the choke coil to which the pulse voltage from the switching element is applied, the first and second voltages are compared with each other by the comparator-detector, and the operation of the switching element is controlled according to the output of the comparator-detector to provide overcurrent protection, wherein the first voltage detected from one end of the choke coil is partially divided by first and second resistors and inputted to one input end of the comparator-detector and wherein the second voltage detected from the other end of the choke coil is integrated by third and fourth resistors and a capacitor and inputted to the other input end of the comparator-detector.

According to another preferred embodiment of the present invention, a switching power supply includes a choke coil, a switching element and a comparator-detector, wherein first and second voltages are detected from both ends of the choke coil to which the pulse voltage from the switching element is applied, the first and second voltages are compared with each other by the comparator-detector, and the operation of the switching element is controlled according to the output of the comparator-detector to implement overcurrent protection, wherein first and second resistors are connected in series between a terminal to detect the first voltage of the choke coil and the ground, the connection point of the first and second resistors is connected to one input end of the comparator-detector, wherein a third resistor and a parallel circuit including a fourth resistor and a capacitor are connected in series between a terminal to detect the second voltage of the choke coil and the ground, the connection point of the third resistor to the fourth resistor is connected to the other input end of the comparator-detector, and wherein the output end of the comparator-detector is connected to a drive circuit operating the switching element.

According to another preferred embodiment of the present invention, a switching power supply accumulates the pulse voltage applied when a switching transistor is switched on as energy and detects the current can be achieved by a single choke coil because the first and second voltages are detected from both ends of the choke coil, the first voltage is partially divided by the first and second resistors and inputted in one input end of the comparator-detector, and the second voltage is integrated by the third and fourth resistors and the capacitor, and inputted in the other input end of the comparator-detector.

Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

FIG. 1 is a circuit diagram of a switching power supply according to a preferred embodiment of the present invention; and

FIG. 2 is a circuit diagram of a conventional switching power supply.

Preferred embodiments of the present invention will be specifically described with reference to the attached drawings.

FIG. 1 is a circuit diagram of a switching power supply according to a preferred embodiment of the present invention. A switching power supply 10 preferably includes a switching element 11 , a diode D 11 , a choke coil L 11 , a comparator-detector 12 , and a drive circuit 13 .

The switching element 11 and the choke coil L 11 are connected between the input Vi and the output Vo, a first resistor R 11 and a second resistor R 12 are connected in series to one end of the choke coil L 11 , i.e., between the output Vo side and the ground, and a connection point of the first and second resistors R 11 and R 12 is connected to one input end of the comparator-detector 12 .

A third resistor R 13 and a parallel circuit including a fourth resistor R 14 and a capacitor C 11 are connected in series between the other end of the choke coil L 11 , i.e., the input Vi side and the ground, and a connection point of the third and fourth resistors R 13 and R 14 is connected to the other input end of the comparator-detector 12 .

In addition, the output end of the comparator-detector 12 is connected to the input end of the drive circuit 13 , and the output end of the drive circuit 13 is connected to the control end of the switching element 11 .

The switching element 11 is switched on or switched off according to the differential voltage between the input Vi and the output Vo. The diode D 11 is preferably a flywheel diode constructed to dissipate the energy (the power) accumulated in the choke coil L 11 when the switching element 11 is switched on if the switching element 11 is switched off.

The choke coil L 11 accumulates the pulse voltage applied thereto as energy when the switching element 11 is switched on, and reduces ripple noise and spike noise. The comparator-detector 12 is inserted between both ends of the choke coil L 11 , detects the voltage across the choke coil L 11 , and controls the drive circuit 13 setting the ON-OFF ratio of the switching element 11 .

The drive circuit 13 outputs the control signal at the pulse so that the output voltage to the load (not shown in the figure) is consistent with the input voltage, and implements the ON-OFF control of the switching element 11 .

The comparator-detector 12 constitutes a latched circuit that preferably includes two transistors Q 1 and Q 2 , one diode D, four resistors R 1 to R 4 , and one Zener diode Z.

Next, the operation of the switching power supply 10 will be described.

The first voltage (the output voltage) is detected from one end of the choke coil L 11 , and the voltage V 1 that is obtained by partially dividing the first voltage by the first and second resistors R 11 and R 12 is inputted to one input end of the comparator-detector 12 .

The second voltage (the pulse voltage from the switching element 11 ) is detected from the other end of the choke coil L 11 , and the voltage V 2 which is obtained by integrating the second voltage by the third and fourth resistors R 13 and R 14 and the capacitor C 11 is inputted to the other input end of the comparator-detector 12 . The voltage V 2 inputted to the other input end of the comparator-detector 12 is integrated by the third and fourth resistors R 13 and R 14 and the capacitor C 11 , and becomes an average DC voltage of the second voltage detected from the other end of the choke coil L 11 .

The voltage V 1 is compared with the voltage V 2 by the comparator-detector 12 . In a normal state in which the output current does not reach the overcurrent, V 2 is below the overcurrent detecting voltage, and no signal is outputted from the output end of the comparator-detector 12 .

On the other hand, in an abnormal state in which the output current is increased and reaches the overcurrent, the current V 2 becomes greater than the overcurrent detection voltage, the signal is outputted from the output end of the comparator-detector 12 , and the drive circuit 13 implements the ON-OFF control of the switching element 11 according to the signal. As a result, in an abnormal state in which the output current reaches the overcurrent, the output voltage and the output current are reduced to realize overcurrent protection.

Next, the case in which a latched circuit is used for the comparator-detector 12 will be specifically described below. In a normal state in which the output current does not reach the overcurrent, the relationship of the voltages is expressed by the inequality, V2<V1+V BE (where V BE is the base-emitter voltage of the transistor Q 1 ), and the transistor Q 1 is switched off. As a result, no signal is outputted from the output end of the comparator-detector 12 .

On the other hand, in an abnormal state in which the output current reaches the overcurrent, the relationship of the voltages is expressed by the inequality/equation, V2≧V1+V BE , and the transistor Q 1 is switched on. As a result, the signal is outputted from the output end of the comparator-detector 12 , and the drive circuit 13 implements the ON-OFF control of the switching element 11 according to the signal. As a result, in an abnormal state in which the output current reaches the overcurrent, the output voltage and the output current are reduced to achieve overcurrent protection.

Since the voltage V 3 is obtained by the Zener diode Z and the resistors R 1 and R 2 , the transistors Q 1 and Q 2 are maintained in an ON-state even when the voltages V 1 and V 2 drop, thus realizing a latching operation. Since the transistor Q 2 is switched on, a feedback control signal FS is shut off, and the operation of the switching element 11 is stopped to prevent the overcurrent from flowing at the output Vo.

According to the switching power supply in accordance with various preferred embodiments of the present invention, the first and second voltages are detected from both ends of the choke coil to which the pulse voltage from the switching element is applied, the first voltage is partially divided by the first and second resistors and inputted to one input end of the comparator-detector, and the second voltage is integrated by the third and fourth resistors and the capacitor and is inputted to the other input end of the comparator-detector. As a result, accumulating the pulse voltage applied as energy when the switching transistor is switched on and detecting the current can be achieved by a single choke coil.

Thus, the manufacturing cost of the switching power supply is greatly reduced, and at the same time, the size thereof is also greatly reduced.

While preferred embodiments of the invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.